Digital watermark embedding method and apparatus, and digital watermark

ABSTRACT

A specific frequency extraction unit extracts a specific frequency component signal from an input image signal, a phase controller and amplitude controller control at least one of the phase and amplitude of the specific frequency component signal in accordance with watermark information, and a watermark information superposition unit superposes the specific frequency component signal on the input image signal to generate an image signal embedded with the watermark information.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation of U.S. patent application Ser. No. 10/327,072,filed Dec. 24, 2002, now U.S. Pat. No. 6,741,723, the entire contents ofwhich are incorporated herein by reference, and which is a ContinuationApplication of PCT Application No. PCT/JP02/04083, Filed Apr. 24, 2002,which was not published under PCT Article 21(2) in English.

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2001-126748, filed Apr. 24,2001; the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a digital watermark embedding methodand apparatus, and a digital watermark detection method and apparatus,which are effective in preventing illegal copies of a digital movingimage signal provided via, e.g., recording media.

2. Description of the Related Art

As apparatuses for recording and playing back digital image data, suchas a digital VTR, DVD (Digital Versatile Disk), and the like haveprevailed, the number of digital moving images that can be played backby these apparatuses are provided. Various digital moving images aredistributed via digital television broadcast via the Internet, broadcastsatellite, communication satellite, and the like, enabling users toenjoy high-quality digital moving images.

It is easy to form high-quality copies from digital moving images on thedigital signal level. Therefore, if some copy protection or copy controlis not applied to digital moving images, there is the danger ofunrestricted formation of copies of digital images. Therefore, illicitcopies of digital images must be prevented, and the number ofgenerations of copies formed by authorized users must be restricted. Forthis purpose, a method of appending information for copy control to eachdigital moving image, and preventing illicit copies or restrictingcopies has been proposed.

As a technique for superposing additional information to a digitalmoving image in such a way, digital watermarking is known. In digitalwatermarking, information such as identification information of thecopyright owner or user of contents, right information of the copyrightowner, use conditions of contents, secret information required uponusing contents, the aforementioned copy control information, or the like(such information will be referred to as watermark informationhereinafter) is embedded in contents of audio data, music data, movingimage data, still image data, or the like, which has been converted intodigital data, so as not to be easy to perceive. By detecting theembedded watermark information from the contents later as needed,copyright protection, including use control and copy control, can beachieved, and further use of the contents is possible.

As a conventional method of digital watermarking, a method that appliesa spread spectrum technique is known. In this method, watermarkinformation is embedded in a digital moving image in the followingsequence.

In a first step, an image signal undergoes spread spectrum by beingmultiplied by a PN (Pseudorandom Noise) sequence.

In a second step, the image signal after spread spectrum undergoesfrequency transformation (e.g., DCT transformation).

In a third step, watermark information is embedded in the image signalby changing the values of specific frequency components.

In a fourth step, the image signal undergoes inverse frequencytransformation (e.g., IDCT transformation).

In a fifth step, the image signal undergoes inversely spread spectrum(the image signal is multiplied by the same PN sequence as in the firststep).

Watermark information is detected in the following sequence, from thedigital moving image, in which the watermark information has beenembedded in the above sequence.

In a sixth step, the image signal undergoes spread spectrum by beingmultiplied by a PN (Pseudorandom Noise) sequence (the same PN sequenceas in the first step).

In a seventh step, the image signal after spread spectrum undergoesfrequency transformation (e.g., DCT transformation).

In an eighth step, the embedded watermark information is extracted fromthe image signal while paying attention to the values of specificfrequency components.

When digital watermarking is applied to digital productions for thepurpose of prevention of illicit use, a characteristic (robustness) thatcan prevent watermark information from being lost or tampered with, anddeliberate attacks which are normally carried out on digital productionsmust be provided to digital watermarking. As attacks that make thewatermark information of a digital image impossible to detect, cut-out,scaling (enlargement/reduction), rotation, and the like of an image areknown.

When an image that has suffered such attacks is input, the conventionaltechnique recovers synchronization of a PN sequence by executing aprocess for estimating a PN sequence used in the first step at the timeof embedding upon detection of watermark information. After that, theprocesses in the sixth through eighth steps are executed to extract theembedded watermark information. However, in order to recoversynchronization of the PN sequence from the image signal alone, a searchmust be conducted by trying a process for detecting watermarkinformation using a plurality of candidates of PN sequences and adoptinga candidate that can be detected satisfactory. For this purpose,problems of increases in arithmetic operation volume and circuit scaleare posed.

It is an object of the present invention to provide a digital watermarkembedding method and apparatus, and a digital watermark detection methodand apparatus, which can detect embedded watermark information againstattacks such as cut-out, scaling, rotation, and the like of an image,without increasing the arithmetic operation volume and circuit scale.

BRIEF SUMMARY OF THE INVENTION

The first aspect of the present invention provides a digital watermarkembedding method for embedding watermark information in an image signal,comprising the steps of: extracting a specific frequency componentsignal from an input image signal; controlling at least one of a phaseand amplitude of the specific frequency component signal in accordancewith watermark information; and outputting an image signal embedded withthe watermark information by superposing the specific frequencycomponent signal, at least one of the phase and amplitude of which hasbeen controlled, on the input image signal.

The second aspect of the present invention provides a digital watermarkdetection method comprising the steps of: extracting a specificfrequency component signal from an input image signal in which watermarkinformation is embedded; controlling at least one of a phase andamplitude of the extracted specific frequency component signal; andextracting the watermark information by making a correlation operationbetween the specific frequency component signal which has undergone atleast one of phase control and amplitude control, and the input imagesignal.

The third aspect of the present invention provides a digital watermarkembedding apparatus comprising: extraction means for extracting aspecific frequency component signal from an input image signal; controlmeans for controlling at least one of a phase and amplitude of theextracted specific frequency component signal in accordance withwatermark information; and superposing means for superposing thespecific frequency component signal, at least one of the phase andamplitude of which has been controlled by the control means, on theinput image signal so as to output an image signal embedded with thewatermark information.

The fourth aspect of the present invention provides a digital watermarkdetection apparatus comprising the steps of: extraction means forextracting a specific frequency component signal from an input imagesignal in which watermark information is embedded; control means forcontrolling at least one of a phase and amplitude of the extractedspecific frequency component signal; and correlation computing means forextracting the watermark information by making a correlation operationbetween the specific frequency component signal, at least one of thephase and amplitude of which has been controlled by the control means,and the input image signal.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a block diagram showing the basic arrangement of a digitalwatermark embedding apparatus according to an embodiment of the presentinvention;

FIG. 2 is a chart for explaining phase shift of a specific frequencysignal by a phase controller in the embodiment of the present invention;

FIG. 3 is a block diagram showing the basic arrangement of a digitalwatermark detection apparatus according to the embodiment of the presentinvention;

FIG. 4 is a block diagram showing another basic arrangement of a digitalwatermark detection apparatus according to the embodiment of the presentinvention;

FIG. 5 is a graph showing an operation example of peak search for across-correlation value and watermark information detection in thedigital watermark detection apparatus according to the embodiment of thepresent invention;

FIG. 6 is a graph showing an operation example of peak search for across-correlation value and watermark information detection in thedigital watermark detection apparatus according to the embodiment of thepresent invention;

FIG. 7 is a block diagram showing an example of a detailed arrangementof the digital watermark embedding apparatus according to the embodimentof the present invention;

FIG. 8 is a block diagram showing another example of a detailedarrangement of the digital watermark embedding apparatus according tothe embodiment of the present invention;

FIG. 9 is a block diagram showing an example of a detailed arrangementof the digital watermark detection apparatus according to the embodimentof the present invention;

FIG. 10 is a waveform chart of respective signals, which shows theoperation of the digital watermark embedding apparatus shown in FIG. 7or 8;

FIG. 11 is a waveform chart of respective signals, which shows theoperation of the digital watermark detection apparatus shown in FIG. 9;

FIG. 12 is a graph showing the operation of peak search for across-correlation value and watermark information detection whenwatermark information in the digital watermark detection apparatus shownin FIG. 9 is (1, 1);

FIG. 13 is a graph showing the operation of peak search for across-correlation value and watermark information detection whenwatermark information in the digital watermark detection apparatus shownin FIG. 9 is (1, 1);

FIG. 14 is a waveform chart of respective signals, which shows theprocess of the digital watermark embedding apparatus shown in FIG. 7 or8 for an image signal of the N-th line;

FIG. 15 is a waveform chart of respective signals, which shows theprocess of the digital watermark embedding apparatus shown in FIG. 7 or8 for an image signal of the (N+1)-th line;

FIG. 16 is a graph for explaining cross-correlation values in thedigital watermark detection apparatus shown in FIG. 9 with respect to awatermarked image signal obtained by the process shown in FIG. 14;

FIG. 17 is a graph for explaining cross-correlation values in thedigital watermark detection apparatus shown in FIG. 9 with respect to awatermarked image signal obtained by the process shown in FIG. 15;

FIG. 18 is a graph showing the watermark information detection operationin the digital watermark detection apparatus shown in FIG. 9 withrespect to a watermarked image signal obtained by the process shown inFIG. 15;

FIG. 19 is a waveform chart of respective signals, which shows anotherprocess of the digital watermark embedding apparatus shown in FIG. 7 or8 for an image signal of the N-th line;

FIG. 20 is a waveform chart of respective signals, which shows anotherprocess of the digital watermark embedding apparatus shown in FIG. 7 or8 for an image signal of the (N+1)-th line;

FIG. 21 is a graph for explaining cross-correlation values in thedigital watermark detection apparatus shown in FIG. 9 with respect to awatermarked image signal obtained by the process shown in FIGS. 19 and20;

FIG. 22 is a graph showing the cross-correlation values and watermarkdetection operation in the digital watermark detection apparatus shownin FIG. 9 when the digital watermark embedding apparatus shown in FIG. 7or 8 embeds a calibration signal together with watermark information;

FIG. 23 is a graph showing another example of the cross-correlationvalues and watermark detection operation in the digital watermarkdetection apparatus shown in FIG. 9 when the digital watermark embeddingapparatus shown in FIG. 7 or 8 embeds a calibration signal together withwatermark information;

FIG. 24 shows a table which is watermark information in the digitalwatermark embedding apparatus shown in FIG. 7 or 8, and is used toencode a binary value into a ternary value;

FIG. 25 shows another table which is watermark information in thedigital watermark embedding apparatus shown in FIG. 7 or 8, and is usedto encode a binary value into a ternary value;

FIG. 26 is a graph showing the watermark information detection operationin the digital watermark detection apparatus shown in FIG. 9 when aplurality of phase shift amounts are set to have arbitrary intervalswhile maintaining a correlation relationship in the digital watermarkembedding apparatus shown in FIG. 7 or 8;

FIG. 27 is a graph showing the watermark information detection operationin the digital watermark detection apparatus shown in FIG. 9 when remarkinformation is additionally written in the digital watermark embeddingapparatus shown in FIG. 7 or 8;

FIG. 28 is a graph showing the watermark information detection operationin the digital watermark detection apparatus shown in FIG. 3 whenwatermark information (1, 1) is embedded depending on whether or not tosuperpose a specific frequency component signal, which has undergonefixed phase shift by four phase shifters, in the digital watermarkembedding apparatus shown in FIG. 1;

FIG. 29 is a graph showing the watermark information detection operationin the digital watermark detection apparatus shown in FIG. 3 whenwatermark information (1, −1) is embedded depending on whether or not tosuperpose a specific frequency component signal, which has undergonefixed phase shift by four phase shifters, in the digital watermarkembedding apparatus shown in FIG. 1;

FIG. 30 is a block diagram showing the basic arrangement of a digitalwatermark embedding apparatus using an amplitude limiter according to anembodiment of the present invention;

FIG. 31 is a block diagram showing the basic arrangement of a digitalwatermark detection apparatus using an amplitude limiter according tothe embodiment of the present invention;

FIG. 32 is a block diagram showing the basic arrangement of a digitalwatermark embedding apparatus using an amplitude limiter according to anembodiment of the present invention;

FIG. 33 is a block diagram showing the basic arrangement of a digitalwatermark detection apparatus using an amplitude limiter according tothe embodiment of the present invention;

FIG. 34 is a block diagram showing the basic arrangement of a digitalwatermark embedding apparatus using randomizing information according toan embodiment of the present invention;

FIG. 35 is a block diagram showing the basic arrangement of a digitalwatermark detection apparatus using randomizing information according tothe embodiment of the present invention;

FIG. 36 is a block diagram showing an example of the detailedarrangement of a specific frequency component extraction unit in FIGS.34 and 35;

FIG. 37 is a block diagram showing another example of the detailedarrangement of a specific frequency component extraction unit in FIGS.34 and 35;

FIG. 38 is a block diagram showing the basic arrangement of a digitalwatermark embedding apparatus using randomizing information according toan embodiment of the present invention;

FIG. 39 is a block diagram showing the basic arrangement of a digitalwatermark detection apparatus using randomizing information according tothe embodiment of the present invention;

FIG. 40 is a block diagram showing an example of the detailedarrangement of a phase controller shown in FIGS. 38 and 39;

FIG. 41 is a block diagram showing the basic arrangement of a digitalwatermark embedding apparatus using randomizing information according toan embodiment of the present invention;

FIG. 42 is a block diagram showing the basic arrangement of a digitalwatermark detection apparatus using randomizing information according tothe embodiment of the present invention;

FIG. 43 is a block diagram showing the basic arrangement of a digitalwatermark embedding apparatus using randomizing information according toan embodiment of the present invention;

FIG. 44 is a block diagram showing the basic arrangement of a digitalwatermark detection apparatus using randomizing information according tothe embodiment of the present invention; and

FIGS. 45A and 45B are block diagrams showing examples of the detailedarrangement of a nonlinear filter in FIGS. 43 and 44.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described hereinafter withreference to the accompanying drawings.

[First Embodiment]

(Basic Arrangement of Digital Watermark Embedding Apparatus)

FIG. 1 is a block diagram showing the basic arrangement of a digitalwatermark embedding apparatus according to an embodiment of the presentinvention. The digital watermark embedding apparatus receives an imagesignal obtained by converting a moving image or still image into digitaldata as an image signal (to be referred to as a to-be-watermarked imagesignal hereinafter) 10 in which watermark information is to be embedded.This to-be-watermarked image signal 10 may contain both a luminancesignal and color difference signals but may contain a luminance signalalone.

The to-be-watermarked image signal 10 is branched into three paths, andis input to a specific frequency component extraction unit 11, featureamount extraction unit 15, and watermark information superposition unit16. The specific frequency component extraction unit 11 comprises adigital filter of the frequency domain, e.g., a bandpass filter having apredetermined cutoff frequency or a bandpass filter having apredetermined passband center frequency, and extracts a specificfrequency component, e.g., a relatively high frequency component, fromthe input moving image signal. In the following description, the outputsignal from the specific frequency component extraction unit 11 will bereferred to as a specific frequency component signal.

The phase and amplitude of the specific frequency component signaloutput from the specific frequency component extraction unit 11 arecontrolled by a phase controller 12 and amplitude controller 13. In thisembodiment, the phase controller 12 is arranged at the former stage, andthe amplitude controller 13 is arranged at the latter stage. However,alternatively, the amplitude controller 13 may be arranged at the formerstage, and the phase controller 12 may be arranged at the latter stage.Watermark information 14 as digital information to be embedded in theinput image signal 10 is supplied to at least one of the phasecontroller 12 and amplitude controller 13.

The phase controller 12 is designed to subject the specific frequencycomponent signal to phase control by a predetermined, unique phasecontrol amount. More specifically, the phase controller 12 isimplemented by one or a plurality of digital phase shifters, and thephase control amount corresponds to a phase shift amount of each phaseshifter. FIG. 2 shows the phase shift process of the phase controller12. In this example, the phase of the specific frequency componentsignal is simply shifted while maintaining its waveform. When thewatermark information 14 is input to the phase controller 12, the phasecontrol amount (phase shift amount) of the phase controller 12 iscontrolled in accordance with the watermark information 14.

The amplitude controller 13 is designed to subject the input specificfrequency component signal to amplitude control by a predetermined,unique amplitude control amount. More specifically, the amplitudecontroller 13 comprises one or a plurality of EX-OR gates and digitalmultipliers, and the amplitude control amount corresponds to acoefficient to be multiplied by the input specific frequency componentsignal. When the watermark information 14 is input to the amplitudecontroller 13, the amplitude control amount (coefficient) of theamplitude controller 13 is controlled in accordance with the watermarkinformation 14.

Furthermore, in this embodiment, the feature amount extraction unit 15extracts a feature amount of the to-be-watermarked image signal 10,e.g., an activity that represents the complexity of an image. Thisfeature amount information is input to the amplitude controller 13. Theamplitude controller 13 controls the amplitude control amount(coefficient) of the specific frequency component in accordance with theinput feature amount. More specifically, when the feature amount is anactivity, a larger coefficient is set with increasing activity. Notethat the feature amount extraction unit 15 is not indispensable, and maybe omitted.

The specific frequency component signal is subjected to phase controland amplitude control by the phase controller 12 and amplitudecontroller 13. This specific frequency component signal is supplied as awatermarking signal and is superposed on the to-be-watermarked imagesignal 10 by the watermark information superposition unit 16 whichcomprises a digital adder. That is, the specific frequency componentsignal extracted by the specific frequency component extraction unit 11is subjected to phase control and amplitude control unique to thedigital watermark embedding apparatus by the phase controller 12 andamplitude controller 13, and one or both the phase control amount andamplitude control amount are controlled in accordance with the watermarkinformation 14. For this reason, the watermark information superpositionunit 16 embeds the watermark information 14 in the to-be-watermarkedimage signal 10. Note that the specific frequency component extractionunit 11 may extract a plurality of channels of specific frequencycomponents, and the phase controller 12 and amplitude controller 13 maycontrol the phases and amplitudes of the plurality of channels ofspecific frequency components to generate a plurality of channels ofspecific frequency component signals. In such a case, the plurality ofchannels of specific frequency component signals are superposed on theto-be-watermarked image signal 10 by the watermark informationsuperposition unit 16.

An image signal (to be referred to as a watermarked image signalhereinafter) 17 in which the watermark information has been embedded inthis way is recorded on a recording medium by a digital imagerecording/playback apparatus such as a DVD system or the like, or istransmitted via a transmission medium such as the Internet, broadcastsatellite, communication satellite, or the like.

(Basic Arrangement of Digital Watermark Detection Apparatus)

The basic arrangement of a digital watermark detection apparatuscorresponding to the digital watermark embedding apparatus shown in FIG.1 will be explained below using FIGS. 3 and 4.

The digital watermark detection apparatus in FIG. 3 receives thewatermarked image signal 17 generated by the digital watermark embeddingapparatus shown in FIG. 1 as an input watermarked image signal 20 via arecording medium or transmission medium. This watermarked image signal20 is branched into three paths, and is input to a specific frequencycomponent extraction unit 21, a feature amount extraction unit 24, andone input of a correlation computation unit 25.

The specific frequency component extraction unit 21 comprises the samehigh-pass filter (HPF) or low-pass filter (LPF) as that in the specificfrequency component extraction unit 11 used in the digital watermarkembedding apparatus shown in FIG. 1. The specific frequency componentextraction unit 21 extracts the same specific frequency component asthat extracted from the watermarked image signal 10 of the watermarkedimage signal 20, by the specific frequency component extraction unit 11.

The phase and amplitude of the specific frequency component signaloutput from the specific frequency component extraction unit 21 arecontrolled by a phase controller 22 and amplitude controller 23. In thisembodiment, the phase controller 22 is arranged at the former stage, andthe amplitude controller 23 is arranged at the latter stage. However,alternatively, the amplitude controller 23 may be arranged at the formerstage, and the phase controller 22 may be arranged at the latter stage.

The phase controller 22 is designed to subject the specific frequencycomponent signal to phase control by a predetermined, unique phasecontrol amount. More specifically, the phase controller 22 isimplemented by a digital phase shifter, as will be described later. Thesame phase control amount (phase shift amount) as that given by thephase controller 12 used in the digital watermark embedding apparatusshown in FIG. 1 is input to the phase controller 22.

The amplitude controller 23 multiplies the specific frequency componentsignal by a coefficient corresponding to a feature amount extracted bythe feature amount extraction unit 24 from the watermarked image signal20, e.g., the activity representing the complexity of an image.

The specific frequency component signal, the phase and amplitude ofwhich have been controlled by the phase controller 22 and amplitudecontroller 23, is input to the other input of the correlationcomputation unit 25. This correlation computation unit 25 makes acorrelation (more specifically, cross-correlation) operation between thespecific frequency component signal and watermarked image signal 20, anddetects embedded watermark information 26. That is, upon observing achange in cross-correlation value with respect to the phase shiftamount, a peak appears at the position of the phase shift amountcorresponding to the phase control amount of the phase controller 22,and the polarity of this peak represents watermark information. The peakof the cross-correlation value assumes either a positive or negativevalue in correspondence with watermark information. For example, if thepeak is positive, it is determined that the watermark information is“1”; if the peak is negative, it is determined that the watermarkinformation is “0”. In this way, the correlation computation unit 25outputs determined watermark information 26.

FIG. 4 shows a digital watermark detection apparatus obtained bymodifying the digital watermark detection apparatus shown in FIG. 3.This digital watermark detection apparatus has an arrangement suitablefor a case wherein the watermarked image signal 20 has suffered scaling.If the watermarked image signal 20 has suffered scaling, the phase shiftamount of the specific frequency component signal assumes a valuedifferent from that given to the specific frequency component signal inthe digital watermark embedding apparatus.

In this embodiment, the phase shift amount of the phase controller 22 iscontrolled continuously or step by step in accordance with phase shiftamount information. Based on this, a watermark information estimator 27arranged on the output side of the correlation computation unit 25searches for a peak of the cross-correlation value output from thecorrelation computation unit 25, and estimates watermark informationfrom the polarity of the found peak. In this example, since thecross-correlation value is positive, it is estimated (determined) thatthe watermark information is “1”.

Assume that the digital watermark embedding apparatus shown in FIG. 1uses a plurality of phase shifters of different phase shift amounts asthe phase controller 12, as will be described later, and a plurality ofamplitude control elements of the amplitude controller 13 are preparedin correspondence with these phase shifters. In such a case, the phasecontroller 22 in FIG. 3 or 4 may comprise a plurality of phase shifters.However, the phase controller 22 may comprise a single phase shifterthat can vary its phase shift amount, and the watermark informationestimator 27 may search for a peak of the cross-correlation value outputfrom the correlation computation unit 25 while changing the phase shiftamount in accordance with phase shift amount information, as shown inFIG. 4. In this case, a peak can be detected in correspondence with thephase shift amount of the phase shifter at the time of embedding thewatermark information, as shown in FIG. 6, and each watermark can beestimated.

EXAMPLE 1 OF DETAILED ARRANGEMENT OF DIGITAL WATERMARK EMBEDDINGAPPARATUS

FIG. 7 shows a detailed embodiment of the digital watermark embeddingapparatus according to the present invention. The correspondence withFIG. 1 that shows the basic arrangement of the digital watermarkembedding apparatus will be explained below. A high-pass filter (HPF)31, n phase shifters (PS) 32-1 to 32-n, n EXOR gates (EXOR) 33-1 to 33-nand multipliers (MPY) 34-1 to 34-n, an activity computation circuit 35,and a digital adder 36 respectively correspond to the specific frequencycomponent extraction unit 11, phase controller 12, amplitude controller13, feature amount extraction unit 15, and watermark informationsuperposition unit 16.

A specific frequency component signal output from the high-pass filter31 is subjected to phase shifts of predetermined different shift amountsby the phase shifters 32-1 to 32-n, and is then input to the inputs onone side of the EXOR gates 33-1 to 33-n. The inputs on the other side ofthe EXOR gates 33-1 to 33-n receive respective bits of n-bit watermarkinformation 14 (CCI). The outputs from the EXOR gates 33-1 to 33-n aremultiplied by an activity computed by the activity computation circuit35 in the multipliers 34-1 to 34-n.

A watermarking signal as the outputs from the multipliers 34-1 to 34-nis added to the to-be-watermarked signal 10 by the adder 36, whereby thewatermark information 14 in the to-be-watermarked signal 10 and thewatermarked image signal 17 is generated.

EXAMPLE 2 OF DETAILED ARRANGEMENT OF DIGITAL WATERMARK EMBEDDINGAPPARATUS

FIG. 8 shows a digital watermark embedding apparatus as a modificationof FIG. 7. In this modification, 3-input multipliers (MPY) 37-1 to 37-nare used in place of the EXOR gates 33-1 to 33-n and multipliers 34-1 to34-n in FIG. 7. The first inputs of the multipliers 37-1 to 37-nrespectively receive the phase-shifted specific frequency componentsignal from the phase shifters 32-1 to 32-n. The second inputs receiverespective bits of the n-bit watermark information 14 (CCI), and thethird inputs commonly receive the activity computed by the activitycomputation circuit 35. With this arrangement as well, functionsequivalent to those of the digital watermark embedding apparatus shownin FIG. 7 can be obtained.

EXAMPLE OF DETAILED ARRANGEMENT OF DIGITAL WATERMARK EMBEDDING APPARATUS

FIG. 9 shows a digital watermark detection apparatus according toanother embodiment of the present invention, and this detectionapparatus corresponds to the digital watermark embedding apparatus shownin FIG. 7. The correspondence between the digital watermark detectionapparatus in FIG. 9 and the basic arrangement of the digital watermarkdetection apparatus in FIG. 4 will be explained below. A high-passfilter 41 corresponds to the specific frequency component extractionunit 21, and n phase shifters (PS) 42-1 to 42-n correspond to the phasecontroller 22. Also, n first multipliers (MPY) 43-1 to 43-n correspondto the amplitude controller 23, and an activity computation circuit 44corresponds to the feature amount extraction unit 24. Furthermore, nsecond multipliers (MPY) 45-1 to 45-n and cumulative adders 46-1 to 46-ncorrespond to the correlation computation unit 25, and a CCI estimator47 corresponds to the watermark information estimator 27.

A specific frequency component signal output from the high-pass filter41 is subjected to phase shifts of predetermined shift amounts, whichare the same as those of the phase shifters 32-1 to 32-n in FIG. 7, bythe phase shifters 42-1 to 42-n, and is then multiplied by the activitycomputed by the activity computation circuit 44 by the first multipliers43-1 to 43-n.

The output signals from the first multipliers 43-1 to 43-n aremultiplied by the watermarked image signal 20 by the second multipliers45-1 to 45-n. The output signals from the second multipliers 45-1 to45-n are cumulatively added by the cumulative adders 45-1 to 45-n, andare then input to the CCI estimator 47, which generates respective bitsof watermark information 26 (CCI).

OPERATION EXAMPLE 1 OF DIGITAL WATERMARK EMBEDDING/DETECTION APPARATUS

A practical operation example executed when the digital watermarkembedding apparatus in FIG. 7 or 8 embeds 2-bit watermark information,and the digital watermark detection apparatus in FIG. 9 detects thatwatermark information will be explained below using FIGS. 10 to 12.

In the digital watermark embedding apparatus in FIG. 7 or 8, thehigh-pass filter 31 extracts a specific frequency component signalindicated by (b) of FIG. 10 from the to-be-watermarked image signal 10indicated by (a) of FIG. 10. The two phase shifters 32-1 and 32-2phase-shift this specific frequency component signal by predeterminedshift amounts. The EXOR gates 33-1 and 33-2 in FIG. 7 or the multipliers37-1 and 37-2 in FIG. 8 multiply these phase shift signals by factorswhich express the 0th and 1st bits of the watermark information 14(CCI), respectively. For example, if the watermark information 14 is“0”, the phase shift signal is multiplied by −1; if it is “1”, the phaseshift signal is multiplied by +1. In FIG. 10, (c) and (d) respectivelyindicate phase shift signals output from the EXOR gates 33-1 and 33-2 ormultipliers 37-1 and 37-2 when watermark information is (1, 1).

Furthermore, the multipliers 34-1 and 34-2 multiply the phase shiftsignals by an activity computed by the activity computation circuit 35as needed. After that, the adder 36 adds the products to theto-be-watermarked image signal 10, thus generating the watermarked imagesignal 17 indicated by (e) of FIG. 10. The solid curve indicated by (e)of FIG. 10 represents the watermarked image signal 17, and a waveformindicated by (a) of FIG. 10 is obtained by mixing the to-be-watermarkedimage signal and the phase shift signals indicated by (c) and (d) inFIG. 10 by addition.

On the other hand, when watermark information is detected by the digitalwatermark detection apparatus shown in FIG. 9 from the watermarked imagesignal (e) embedded with the watermark information, as shown in FIG. 10,the high-pass filter 41 extracts a specific frequency component signalindicated by (b) of FIG. 11 from the watermarked image signal 20indicated by (a) of FIG. 11 (corresponding to the watermarked imagesignal 17 indicated by (e) of FIG. 10). When the watermarked imagesignal 20 has not suffered any scaling, the phase shifters 42-1 and 42-2phase-shift the watermarked image signal 20 by the same predeterminedshift amounts as those of the phase shifters 32-1 and 32-2 in FIG. 7, asindicated by (c) and (d) of FIG. 11.

The first multipliers 43-1 and 43-2 multiply an activity in accordancewith the phase shift signals indicated by (c) and (d) of FIG. 11. Afterthat, the second multipliers 45-1 and 45-2 multiply the watermarkedimage signal 20 indicated by (a) of FIG. 11 by the outputs from thefirst multipliers 43-1 and 43-2. The cumulative adders 46-1 and 46-2cumulatively add the products from the second multipliers 45-1 and 45-2.In this way, the cross-correlation values of the phase shift signals areobtained, and watermark information is determined from peaks of thecross-correlation values. For example, if the peak of thecross-correlation value is positive, it is determined that the watermarkinformation is +1 (“1”); if the peak of the cross-correlation value isnegative, it is determined that the watermark information is −1 (“0”).

On the other hand, if the watermarked image signal 20 has sufferedscaling, the phase shift amounts of the phase shifters 42-1 and 42-2 arecontrolled in the same manner as in FIG. 4 to search for phase shiftamounts. That is, the CCI estimator 47 searches for peaks of thecross-correlation values upon controlling the phase shift amount, andestimates the watermark information 26 from the peak positions.

For example, if watermark information 14 (CCI) is (1, 1), two positivepeaks of cross-correlation values are present in addition to the origin(a point where the phase shift amount is zero), as shown in FIG. 12,thus determining watermark information.

On the other hand, if the watermark information 14 (CCI) is (1, −1), apositive peak of a cross-correlation value is present at a position nearthe origin, and a negative peak is present at a position farther fromthe origin than the positive peak, as shown in FIG. 13, thus determiningwatermark information.

OPERATION EXAMPLE 2 OF DIGITAL WATERMARK EMBEDDING/DETECTION APPARATUS

Another operation example of the digital watermark embedding apparatusin FIG. 7 or 8 and the digital watermark detection apparatus in FIG. 9will be described below using FIGS. 14 to 18. In this method, thedigital watermark embedding apparatus inverts the polarity of each phaseshift signal by one of every line, every set of a plurality of lines,every field, every set of a plurality of fields, every frame, and everyset of a plurality of frames, or appropriate combinations of them. Anoperation example when watermark information consists of 2 bits will beexplained below.

The digital watermark embedding apparatus executes a process shown inFIG. 14 for the N-th line (N=1, 2, . . . ) of the to-be-watermarkedimage signal 10.

The two phase shifters 32-1 and 32-2 phase-shift a specific frequencycomponent signal, which is extracted by the high-pass filter 31 from theN-th line signal indicated by (a) of FIG. 14 of the to-be-watermarkedimage signal 10, by predetermined shift amounts. The EXOR gates 33-1 and33-2 in FIG. 7 or the multipliers 37-1 and 37-2 in FIG. 8 respectivelymultiply these phase shift signals by factors which express the 0th and1st bits of the watermark information 14 (CCI). For example, if thewatermark information 14 is “0”, the phase shift signal is multiplied by−1; if the watermark information is “1”, the phase shift signal ismultiplied by +1. In FIG. 14, (b) and (c) respectively indicate phaseshift signals output from the EXOR gates 33-1 and 33-2 or multipliers37-1 and 37-2 when watermark information is (1, 1).

Furthermore, the multipliers 34-1 and 34-2 multiply the phase shiftsignals by an activity computed by the activity computation circuit 35as needed. After that, the adder 36 adds the products to theto-be-watermarked image signal 10. As a result, the to-be-watermarkedimage signal indicated by the broken curve in (d) of FIG. 14(corresponding to waveform (a) of FIG. 14), and the phase shift signalsindicated by (b) and (c) in FIG. 14 are mixed by addition, thusgenerating the watermarked image signal 17 indicated by the solid curve.

Subsequently, the digital watermark embedding apparatus executes aprocess shown in FIG. 15 for the (N+1)-th line of the to-be-watermarkedimage signal 10.

The two phase shifters 32-1 and 32-2 phase-shift a specific frequencycomponent signal, which is extracted by the high-pass filter 31 from the(N+1)-th line signal indicated by (a) of FIG. 15 of theto-be-watermarked image signal 10, by predetermined shift amounts. TheEXOR gates 33-1 and 33-2 in FIG. 7 or the multipliers 37-1 and 37-2 inFIG. 8 respectively multiply these phase shift signals by factors whichexpress the 0th and 1st bits of the watermark information 14 (CCI). Inthis case, contrary to the case for the N-th line signal, for example,if the watermark information 14 is “0”, the phase shift signal ismultiplied by +1; if it is “1”, the phase shift signal is multiplied by−1. Therefore, the polarities of the phase shift signals output from theEXOR gates 33-1 and 33-2 or multipliers 37-1 and 37-2 when the watermarkinformation is (1, 1) are inverted, as indicated by (b) and (c) in FIG.15, unlike the waveforms indicated by (b) and (c) of FIG. 14.

Furthermore, the multipliers 34-1 and 34-2 multiply the phase shiftsignals by an activity computed by the activity computation circuit 35as needed. After that, the adder 36 adds the products to theto-be-watermarked image signal 10, thereby generating the watermarkedimage signal 17 indicated by the solid curve (in (d) of FIG. 15)obtained by mixing the to-be-watermarked image signal indicated by thebroken curve in (d) of FIG. 15 (corresponding to waveform (a) of FIG.15), and the phase shift signals indicated by (b) and (c) in FIG. 15 byaddition.

In the above description, the polarities of the phase shift signals areinverted between the N-th and (N+1)-th lines of the to-be-watermarkedimage signal, i.e., every line, but may be inverted every set of aplurality of lines, every field, every set of a plurality of fields,every frame, or every set of a plurality of frames.

On the other hand, the digital watermark detection apparatus in FIG. 9inverts the polarities upon cumulative addition as needed incorrespondence with polarity inversion of the phase shift signals by oneof every line, every set of a plurality of lines, every field, every setof a plurality of fields, every frame, and every set of a plurality offrames, or appropriate combinations of them. For example, when thepolarities of the phase shift signals have been inverted every line, ashas been explained using FIGS. 14 and 15, positive peaks of thecross-correlation values for the N-th line of the watermarked imagesignal 20 appear in correspondence with the phase shift amounts, asshown in FIG. 16. However, negative peaks of the cross-correlationvalues for the (N+1)-th line of the watermarked image signal 20 appearin correspondence with the phase shift amounts, as shown in FIG. 17.Hence, the polarities of the cross-correlation values output from themultipliers 45-1 and 45-2 are inverted every line, and these values arecumulatively added by the cumulative adders 46-1 and 46-2. In this case,since positive peaks of the cross-correlation values after cumulativeaddition appear successively, as shown in FIG. 18, it is determined thatthe watermark information is (1, 1).

In this way, polarity inversions of the phase shift signals are combinedupon embedding watermark information, and the cross-correlation valuesare cumulatively added after their polarities are inverted accordingly.As a result, the watermark information can be rendered imperceptible onthe image, and tampering of watermark information can be prevented moreeffectively.

OPERATION EXAMPLE 3 OF DIGITAL WATERMARK EMBEDDING/DETECTION APPARATUS

A further operation example of the digital watermark embedding apparatusin FIG. 7 or 8 and the digital watermark detection apparatus in FIG. 9will be described below using FIGS. 19 to 21. In this method, thedigital watermark embedding apparatus inverts phase shift amounts in theright-and-left direction for every line, and an operation when watermarkinformation consists of 2 bits will be explained below.

The digital watermark embedding apparatus executes a process shown inFIG. 19 for the N-th line (N=1, 2, . . . ) of the to-be-watermarkedimage signal 10.

The two phase shifters 32-1 and 32-2 phase-shift a specific frequencycomponent signal, which is extracted by the high-pass filter 31 from theN-th line signal indicated by (a) of FIG. 19 of the to-be-watermarkedimage signal 10, to the right, i.e., in a phase lead direction bypredetermined shift amounts. The EXOR gates 33-1 and 33-2 in FIG. 7 orthe multipliers 37-1 and 37-2 in FIG. 8 respectively multiply thesephase shift signals by factors which express the 0th and 1st bits of thewatermark information 14 (CCI) in the same manner as in the abovedescription. In FIG. 19, (b) and (c) respectively indicate phase shiftsignals output from the EXOR gates 33-1 and 33-2 or multipliers 37-1 and37-2 when watermark information is (1, 1).

Furthermore, the multipliers 34-1 and 34-2 multiply the phase shiftsignals by an activity computed by the activity computation circuit 35as needed. After that, the adder 36 adds the products to theto-be-watermarked image signal 10. As a result, the to-be-watermarkedimage signal indicated by the broken curve in (d) of FIG. 19(corresponding to waveform (a) of FIG. 19), and the phase shift signalsindicated by (b) and (c) in FIG. 19 are mixed by addition, thusgenerating the watermarked image signal 17 with a waveform indicated bythe solid curve.

On the other hand, the digital watermark embedding apparatus executes aprocess shown in FIG. 20 for the (N+1)-th line of the to-be-watermarkedimage signal 10.

The two phase shifters 32-1 and 32-2 phase-shift a specific frequencycomponent signal, which is extracted by the high-pass filter 31 from theN-th line signal indicated by (a) of FIG. 20 of the to-be-watermarkedimage signal 10, to the left, i.e., in a phase lag direction bypredetermined shift amounts. The EXOR gates 33-1 and 33-2 in FIG. 7 orthe multipliers 37-1 and 37-2 in FIG. 8 respectively multiply thesephase shift signals by factors which express the 0th and 1st bits of thewatermark information 14 (CCI) in the same manner as in the abovedescription. In FIG. 20, (b) and (c) respectively indicate phase shiftsignals output from the EXOR gates 33-1 and 33-2 or multipliers 37-1 and37-2 when watermark information is (1, 1).

Furthermore, the multipliers 34-1 and 34-2 multiply the phase shiftsignals by an activity computed by the activity computation circuit 35as needed. After that, the adder 36 adds the products to theto-be-watermarked image signal 10. As a result, the to-be-watermarkedimage signal indicated by the broken curve in (d) of FIG. 20(corresponding to waveform (a) of FIG. 20), and the phase shift signalsindicated by (b) and (c) in FIG. 20 are mixed by addition, thusgenerating the watermarked image signal 17 with a waveform indicated bythe solid curve.

On the other hand, the digital watermark detection apparatus in FIG. 9simply cumulatively adds the cross-correlation values every line tosearch for peaks, thereby detecting watermark information. However, asdescribed in operation example 2, when the polarities of the phase shiftsignals have been inverted by one of every line, every set of aplurality of lines, every field, every set of a plurality of fields,every frame, and every set of a plurality of frames, or appropriatecombinations of them, polarity inversion is also made for eachcumulative addition unit.

FIG. 21 shows cross-correlation values after cumulative addition everyline in this case. Upon searching for phase shift amounts by settingthem in the positive and negative directions, cross-correlation valueshaving nearly the same patterns, i.e., cross-correlation valuesax-symmetrical about the center, can be obtained. By exploiting such acharacteristic of the cross-correlation values, watermark informationcan be detected by conducting a search in only one direction (e.g.,right direction).

OPERATION EXAMPLE 4 OF DIGITAL WATERMARK EMBEDDING/DETECTION APPARATUS

A further operation example of the digital watermark embedding apparatusin FIG. 7 or 8 and the digital watermark detection apparatus in FIG. 9will be described below using FIGS. 22 to 25. An example described belowis a method of embedding a calibration signal together upon embeddingwatermark information, and using that calibration signal in detection ofthe watermark information. A practical operation example will beexplained below.

(1) Upon embedding N-bit watermark information in an image signal, thedigital watermark embedding apparatus generates (N+1)-bit phase shiftsignals, and embeds 1 bit other than N bits used to embed watermarkinformation in each phase shift signal as a calibration signal, so thatthe 1 bit always has level +1 (or −1). This calibration signal serves asa reference upon detecting watermark information.

On the other hand, the digital watermark detection apparatus detectswatermark information based on correlation between the cross-correlationvalues at respective positions of phase shifts, and those correspondingto the calibration signal, since that correlation is known. For example,assuming that a calibration signal is embedded as level +1 (or −1) ineach phase shift signal, it is estimated that information is +1 (or −1)when a cross-correlation value at a position corresponding to thecalibration signal, and that at another embedded position have the samepolarity, or it is estimated that information is −1 (or +1) when theyhave different polarities, as shown in FIG. 22.

(2) The digital watermark embedding apparatus may embed a calibrationsignal at a position where the phase shift amount is minimum or maximum.In such a case, the digital watermark detection apparatus detects thecalibration signal at the position where the phase shift amount isminimum or maximum, and determines the embedded watermark informationbased on correlation between that calibration signal and watermarkinformation.

(3) The digital watermark embedding apparatus embeds a calibrationsignal at a position where the phase shift amount is minimum or maximumas a predetermined value (e.g., +1 or −1), sets a plurality of phaseshift amounts at equal intervals, and embeds ternary information {+1, 0,−1} at respective phase shift positions.

More specifically, for example, when ternary information is {+1}, aphase shift signal multiplied by a positive multiplier is added to theto-be-watermarked image signal 10; when ternary information is {−1}, aphase shift signal multiplied by a negative multiplier is added to theto-be-watermarked image signal 10; and when ternary information is {0},nothing is added to the to-be-watermarked image signal 10.

On the other hand, the digital watermark detection apparatus obtainscross-correlation values at the phase shift positions of the watermarkinformation, which are estimated from the calibration signal. As shownin FIG. 23, when this cross-correlation value is in the neighborhood ofzero, it is determined that ternary information is {0}; when thecross-correlation value is not in the neighborhood of zero, it isdetermined based on correlation between the cross-correlation value atthe phase shift position of the watermark information and that of thecalibration signal that ternary information is {+1, −1}. Applicationexamples of operation example (3) will be described below.

(3-1) The digital watermark embedding apparatus encodes binary values toternary values as watermark information, as shown in FIG. 24, and embedsthese ternary values as ternary information, as described above.

The digital watermark detection apparatus decodes ternary valuesdetected as ternary information, and detects watermark information oforiginal binary values.

(3-2) As in the above example, the digital watermark embedding apparatusencodes binary values to ternary values as watermark information, andembeds these ternary values as ternary information, as is describedabove. In this case, a combination of all “0”s is not used asinformation to be embedded, as shown in FIG. 25.

The digital watermark detection apparatus decodes ternary valuesdetected as ternary information, and detects watermark information oforiginal binary values.

(3—3) A ternary value is used as a carry for CCI.

(3-4) Watermark information is embedded and detected while setting CopyFree (can be copied unlimitedly) to be +1, Copy Once (can be copied onlyonce) to be 0, and Never Copy (cannot be copied) to be −1. In this case,since −1 is embedded at a position of “0” in Remark of watermarkinformation, the need for cancel can be obviated.

OPERATION EXAMPLE 5 OF DIGITAL WATERMARK EMBEDDING/DETECTION APPARATUS

A further operation example of the digital watermark embedding apparatusin FIG. 7 or 8 and the digital watermark detection apparatus in FIG. 9will be described below. The digital watermark embedding apparatus setsa plurality of phase shift amounts at arbitrary intervals withoutdisturbing their correlation. In this case, the digital watermarkdetection apparatus counts the number of peaks of cross-correlationvalues, and determines in such a manner that the innermost peak, whichis closest to the origin, is bit 0, the next innermost peak is bit 1, .. . , as shown in FIG. 26.

OPERATION EXAMPLE 6 OF DIGITAL WATERMARK EMBEDDING/DETECTION APPARATUS

A further operation example of the digital watermark embedding apparatusin FIG. 7 or 8 and the digital watermark detection apparatus in FIG. 9will be described below using FIG. 27. The digital watermark embeddingapparatus detects the outermost one of existing embedded positions wherewatermark information is embedded, and additionally writes informationfor Remark outside the detected position.

On the other hand, the digital watermark detection apparatus searchesuntil all peaks of cross-correlation values are found, and determinesinformation after Remark based on information embedded at the outermostposition.

OPERATION EXAMPLE 7 OF DIGITAL WATERMARK EMBEDDING/DETECTION APPARATUS

The operations of the digital watermark embedding apparatus in FIG. 1upon controlling the phase controller 12 using the watermark information14, and the digital watermark detection apparatus shown in FIG. 3 or 4will be described below.

The phase controller 12 comprises, e.g., four phase shifters havingdifferent phase shift amounts, and a switch used to select these phaseshifters. Assume that a specific frequency component signal from thespecific frequency component extraction unit 11 is in parallel input tothese phase shifters. If θ1, θ2, θ3, and θ4 respectively represent thephase shift amounts of the four phase shifters, for example, the 0th bitof the watermark information 14 is expressed by the presence/absence ofsuperposition between specific frequency component signals which havebeen phase-shifted by the shift amounts θ1 and θ2. The 1st bit of thewatermark information 14 is expressed by the presence/absence ofsuperposition between specific frequency component signals which haveundergone phase shifts of the shift amounts θ3 and θ4. Morespecifically, the specific frequency component signal is superposed onthe to-be-watermarked image signal 10 in accordance with a combinationof (a-1) and (a-2) or a combination of (b-1) and (b-2), explained below.

(a-1) If the 0th bit=“1”, only a specific frequency component signalwhich has been phase-shifted by θ1 is superposed on theto-be-watermarked image signal, and a specific frequency componentsignal which has been phase-shifted by θ2 is not superposed on theto-be-watermarked image signal.

(a-2) If the 1st bit=“1”, only a specific frequency component signalwhich has been phase-shifted by θ3 is superposed on theto-be-watermarked image signal, and a specific frequency componentsignal which has been phase-shifted by θ4 is not superposed on theto-be-watermarked image signal.

(b-1) If the 0th bit=“1”, only a specific frequency component signalwhich has been phase-shifted by θ1 is superposed on theto-be-watermarked image signal, and a specific frequency componentsignal which has been phase-shifted by θ2 is not superposed on theto-be-watermarked image signal.

(b-2) If the 1st bit=“1”, only a specific frequency component signalwhich has been phase-shifted by θ4 is superposed on theto-be-watermarked image signal, and a specific frequency componentsignal which has been phase-shifted by θ3 is not superposed on theto-be-watermarked image signal.

On the other hand, if scaling of the input watermarked image signal 20is not considered, the digital watermark detection apparatus shown inFIG. 3 sets the same phase shift amounts of four phase shifters, whichform the phase controller 22, as the phase shift amounts θ1, θ2, θ3, andθ4 of the phase controller 12 in the digital watermark embeddingapparatus, and determines watermark information on the basis ofcross-correlation values at the phase shift amounts θ1, θ2, θ3, and θ4.

FIG. 28 shows cross-correlation values when watermark information is (1,1), and FIG. 29 shows cross-correlation values when watermarkinformation is (1, −1). The watermark information can be determinedbased on the cross-correlation values at the positions of the phaseshift amounts θ1, θ2, θ3, and θ4.

When scaling of the watermarked image signal 20 is taken intoconsideration, the phase shift amounts given by the digital watermarkembedding apparatus can be searched for by changing the phase shiftamount, and the cross-correlation values can be determined.

[Second Embodiment]

Another embodiment of the present invention will be described belowusing FIGS. 30 to 33. In this embodiment, an amplitude limiter isinserted in a digital watermark embedding apparatus and digitalwatermark detection apparatus. The amplitude limiter limits theamplitude of a signal to be superposed on the to-be-watermarked imagesignal 10. With this process, watermark information is evenly embeddedover a broad level range from the low to high levels of theto-be-watermarked image signal 10. As a result, the image quality can beprevented from deteriorating more effectively.

In a digital watermark embedding apparatus shown in FIG. 30, anamplitude limiter 18 is inserted between the specific frequencycomponent extraction unit 11 and phase & amplitude controller (phasecontroller 12 in this example). In a digital watermark detectionapparatus shown in FIG. 31, an amplitude limiter 28 is inserted betweenthe specific frequency component extraction unit 21 and controller(phase controller 22 in this example) in correspondence with FIG. 30.

In a digital watermark embedding apparatus shown in FIG. 32, anamplitude limiter 18 is inserted between the phase & amplitudecontroller (amplitude controller 13 in this example) and watermarkinformation superposition unit 16. In a digital watermark detectionapparatus shown in FIG. 33, an amplitude limiter 28 is inserted betweenthe phase & amplitude controller (amplitude controller 23 in thisexample) and correlation computation unit 25 in correspondence with FIG.32.

[Third Embodiment]

A further embodiment of the present invention will be described belowusing FIGS. 34 to 42. This embodiment embeds and detects watermarkinformation depending on randomizing information. With this process,watermark information cannot be detected unless randomizing informationis known. As a result, digital watermarking more robust against attackscan be provided.

The randomizing information may be generated inside the digitalwatermark embedding apparatus or detection apparatus, or may be inputfrom an external apparatus as long as the security can be assured. Therandomizing information may be constant or may be changed during adigital watermark embedding process or detection process. For example,randomizing information may be changed in such a manner that differentpieces of randomizing information are used in the left half (former halfof one horizontal scanning period) and right half (latter half of onehorizontal scanning period) in one line of an image signal, or differentpieces of randomizing information are used every line.

In an example of a digital watermark embedding apparatus shown in FIG.34, when a filter which forms the specific frequency componentextraction unit 11 has different characteristics depending parameters,the parameters are given using secret randomizing information 19.

In a digital watermark detection apparatus shown in FIG. 35, parametersof a filter, which forms the specific frequency component extractionunit 21, are given using randomizing information 29 in correspondencewith FIG. 34. The randomizing information 29 is the same as therandomizing information 19 used in the digital watermark embeddingapparatus in FIG. 34. Only the digital watermark detection apparatuswhich can internally generate or externally receive this randomizinginformation 29 can normally detect watermark information 26.

FIGS. 36 and 37 show examples of filters used in the specific frequencycomponent extraction units 11 and 21 in FIGS. 34 and 35. This filtermultiplies successive pixel values { . . . p(h-1), p(h), p(h+1), . . . }of a to-be-watermarked image signal by coefficients, and calculates thesum of these products as a filter output. Since the coefficients can berandomized within a given range, these coefficients are used as therandomizing information 19.

In a digital watermark embedding apparatus shown in FIG. 38, phase shiftamounts of phase shifters, which form the phase controller 12, arerandomized in accordance with the randomizing information 19. In thismanner, the peak pattern of an autocorrelation value is blunted to makethe peak harder to see. In this case, it is desirable to frequentlychange the randomized phase shift amount. For example, different phaseshift amounts are set in the left and right halves of a frame. Also, aframe may be divided into a plurality of strip-shaped regions extendingin the vertical direction, and different phase shift amounts may be setfor respective regions.

In a digital watermark detection apparatus shown in FIG. 39, phase shiftamounts of phase shifters, which form the phase controller 22, arerandomized in accordance with the randomizing information 29 incorrespondence with the digital watermark embedding apparatus shown inFIG. 38. The randomizing information 29 is the same as the randomizinginformation 19 used in the digital watermark embedding apparatus in FIG.38. Only the digital watermark detection apparatus which can internallygenerate or externally receive this randomizing information 29 cannormally detect watermark information 26.

FIG. 40 shows an example of a phase shifter which is used in the phasecontrollers 11 and 21 in FIGS. 38 and 39, and has a variable phase shiftamount. This phase shifter has an arrangement in which a plurality ofphase shift elements are connected in series, and signals fromrespective taps (inputs/outputs of phase shift elements) are selected bya selector in accordance with randomizing information.

In a digital watermark embedding apparatus shown in FIG. 41, anamplitude modulator 51 is inserted between the phase & amplitudecontroller (amplitude controller 13 in this example) and watermarkinformation superposition unit 16, and modulates the amplitude of anembedding signal in accordance with randomizing information 19.

In a digital watermark detection apparatus shown in FIG. 42, anamplitude modulator 61 is inserted between the phase & amplitudecontroller (amplitude controller 23 in this example) and correlationcomputation unit 25 in correspondence with the digital watermarkembedding apparatus in FIG. 41, and modulates the amplitude of anembedding signal in accordance with randomizing information 29. Therandomizing information 29 is the same as the randomizing information 19used in the digital watermark embedding apparatus in FIG. 41. Only thedigital watermark detection apparatus which can internally generate orexternally receive this randomizing information 29 can normally detectwatermark information 26.

In a digital watermark embedding apparatus shown in FIG. 43, a nonlinearfilter 52 is inserted between the phase & amplitude controller(amplitude controller 13 in this example) and watermark informationsuperposition unit 16. The nonlinear filter 52 reduces correlationbetween an embedding signal and to-be-watermarked image signal 10, thuspreventing peaks from appearing in autocorrelation values.

In a digital watermark detection apparatus shown in FIG. 44, a linearfilter 62, which has characteristics opposite to those of the linearfilter 52 used in the digital watermark embedding apparatus in FIG. 43,is inserted between the phase & amplitude controller (amplitudecontroller 23 in this example) and correlation computation unit 25 incorrespondence with the digital watermark embedding apparatus in FIG.43.

As the nonlinear filter 52, a filter that uses amplitude modulationbased on a trigonometric function or an equation of higher degree can beused. This filter is a nonlinear filter that outputs sin(x), x2, or thelike if an input signal is x. FIG. 45 shows an example of the nonlinearfilter 52.

FIG. 45A shows a multiplication device which is formed by giving anidentical input signal to two inputs of a multiplier, and outputs x2when an input signal is x. An overflow part as a result of calculating asquare is removed. FIG. 45B shows a nonlinear filter which implementsthe relationship between the input and output values in the form of atable so as to be able to express complicated nonlinear conversion. Forexample, if this nonlinear conversion table is a sin table, a nonlinearfilter which outputs sin(ax) in response to an input signal x can beimplemented.

As described above, according to the present invention, since a specificfrequency component signal is extracted from an input image signal, atleast one of the phase and amplitude of this specific frequencycomponent signal is controlled in accordance with watermark information,and an image signal embedded with the watermark information can begenerated by superposing the specific frequency component signal, atleast one of the phase and amplitude of which has been controlled, onthe input image signal, a digital watermark embedding method andapparatus, and a digital watermark detection method and apparatus, whichare effective in preventing illegal copies of digital moving imagesignals provided via, e.g., recording media can be implemented.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A digital watermark embedding method of embedding watermarkinformation in an image signal, comprising: extracting a specificfrequency component signal having a phase and an amplitude from theinput image signal; controlling at least the phase of the specificfrequency component signal in accordance with the watermark informationto produce at least one phase-controlled specific frequency componentsignal; outputting an image signal embedded with the watermarkinformation by superposing the phase-controlled specific frequencycomponent signal on the input image signal; and limiting an amplitude ofthe specific frequency component signal.
 2. The digital watermarkembedding method according to claim 1, wherein extracting the specificfrequency component signal includes extracting the specific frequencycomponent signal in random.
 3. The digital watermark embedding methodaccording to claim 1, wherein outputting the image signal comprisessubjecting the phase-controlled specific frequency component signal to anonlinear process, and superposing the linear-processed specificfrequency component signal on the image input signal.
 4. A digitalwatermark embedding method of embedding watermark information in animage signal, comprising: extracting a specific frequency componentsignal containing a phase and an amplitude from the input image signal;shifting the phase of the specific frequency component signal inaccordance with the watermark information to produce at least onephase-shifted specific frequency component signal; outputting an imagesignal embedded with the watermark information by superposing thephase-shifted specific frequency component signal on the input imagesignal; and limiting an amplitude of the specific frequency componentsignal.
 5. The digital watermark embedding method according to claim 4,wherein extracting the specific frequency component signal includesextracting the specific frequency component signal in random.
 6. Adigital watermark detection method comprising: extracting a specificfrequency component signal containing a phase and an amplitude from aninput image signal in which watermark information is embedded;controlling at least the phase of the specific frequency componentsignal extracted to obtain a phase-controlled specific frequencycomponent signal; and performing a correlation operation between thephase-controlled specific frequency component signal and the input imagesignal to extract the watermark information.
 7. The digital watermarkdetection method according to claim 6, further comprising limiting theamplitude of the specific frequency component signal.
 8. The digitalwatermark detection method according to claim 6, further comprisingrandomizing the specific frequency component signal extracted.
 9. Thedigital watermark detection method according to claim 6, whereinperforming the correlation comprises subjecting the phase-controlledspecific frequency component signal to a nonlinear process, andperforming the correlation operation between the image input signal andthe phase-controlled specific frequency component signal subjected tothe nonlinear process.
 10. A digital watermark embedding apparatus whichembeds watermark information in an input image signal, comprising: anextraction unit configured to extract a specific frequency componentsignal containing a phase and an amplitude from the input image signal;a control unit configured to control at least the phase of the extractedspecific frequency component signal in accordance with the watermarkinformation to produce a phase-controlled specific frequency componentsignal; and a superposing unit configured to superpose thephase-controlled specific frequency component signal on the input imagesignal to output an image signal embedded with the watermarkinformation.
 11. The digital watermark embedding apparatus according toclaim 10, further comprising an amplitude limiter which is insertedbetween the extraction unit and the superposing unit and limits theamplitude of the specific frequency component signal.
 12. The digitalwatermark embedding apparatus according to claim 10, wherein acharacteristic of at least one of the extraction unit and the controlunit is randomized using randomizing information.
 13. The digitalwatermark embedding apparatus according to claim 10, further comprisinga nonlinear filter inserted between the control unit and the superposingunit.
 14. A digital watermark detection apparatus which detectswatermark information embedded in an input image signal, comprising: anextraction unit configured to extract a specific frequency componentsignal containing a phase and an amplitude from the input image signal;a control unit configured to control at least the phase of the specificfrequency component signal extracted to produce a phase-controlledspecific frequency component signal; and a correlation computing unitconfigured to perform a correlation operation between the specificfrequency component signal and the input image signal to extract thewatermark information.
 15. The digital watermark detection apparatusaccording to claim 14, further comprising an amplitude limiter which isinserted between the extraction unit and the correlation computing unitand limits the amplitude of the specific frequency component signal. 16.The digital watermark detection apparatus according to claim 14, whereina characteristic of at least one of the extraction unit and the controlunit is randomized using randomizing information.
 17. The digitalwatermark detection apparatus according to claim 14, further comprisinga nonlinear filter inserted between the control unit and the correlationcomputing unit.
 18. A digital watermark embedding apparatus comprising:extraction means for extracting a specific frequency component signalcontaining a phase and an amplitude from an input image signal; controlmeans for controlling at least the phase of the extracted specificfrequency component signal in accordance with watermark information toproduce a phase-controlled specific frequency component signal; andsuperposing means for superposing the phase-controlled specificfrequency component signal on the input image signal to output an imagesignal embedded with the watermark information.
 19. A digital watermarkembedding apparatus according to claim 18, further comprising a limiterinserted between the extraction means and the superposing means andconfigured to limit the amplitude of the specific frequency componentsignal.
 20. A digital watermark embedding apparatus according to claim18, further comprising a nonlinear filter inserted between the controlmeans and the superposing means.
 21. A digital watermark detectionapparatus comprising: extraction means for extracting a specificfrequency component signal containing a phase and an amplitude from aninput image signal in which watermark information is embedded; controlmeans for controlling at least the phase of the extracted specificfrequency component signal to produce at least one phase-controlledspecific frequency component signal; and correlation computing means forperforming a correlation operation between the phase-controlled specificfrequency component signal and the input image signal to extract thewatermark information.